Quantitation of Enantiomeric Excess from External Cavity Quantum Cascade VCD Spectra

Abstract

Vibrational circular dichroism (VCD) is an established technique for the analysis of chiral samples [1]. Currently, VCD is mainly employed for structure elucidation and identification of enantiomers, while its broad application for routine analysis mainly being impeded by the long measurement times (hours) typically associated with VCD spectral acquisition. Our recently developed balanced detection external cavity quantum cascade laser (EC-QCL) based VCD device allows to acquire VCD spectra of several hundred wavenumber wide spectral ranges at signal to noise ratios comparable to state-of-the-art devices within a few minutes. This improved throughput enables the use of VCD as a tool for quantitative determination of enantiomeric excess (EE). In this work we investigate chemometric approaches for EE determination from EC-QCL-VCD using R/S-1,1′-Bi-2-naphthol (BINOL) in CHCl3 as a test case (see Figure 1). Our results show that careful preprocessing and variable selection (i.e. spectral range selection) are key to achieve low uncertainties when applying chemometrics to VCD data.